Heretofore, a lithographic printing plate is obtained by a plate-making method wherein a photosensitive lithographic printing plate precursor having a construction comprising a photosensitive resin layer provided on a support having a hydrophilic surface is ordinarily subjected to floodwise exposure (mask exposure) via a lith film and then the non-image area is removed with a developer. However, with the recent progress of digitized techniques, a computer-to-plate (CTP) technique of directly conducting exposure process on the surface of the lithographic printing plate precursor by scanning according to digitized image data with highly convergent light, for example, a laser beam, without using a lith film has been developed. Photosensitive lithographic printing plate precursors adapted for the technique have also been developed.
As the photosensitive lithographic printing plate precursor suitable for exposure with a laser beam, a photosensitive lithographic printing plate precursor having a polymerizable photosensitive layer is exemplified. Such a photosensitive lithographic printing plate precursor is preferable because the polymerizable photosensitive layer is easily enable to increase sensitivity by appropriately selecting a photopolymerization initiator or a photopolymerization initiation system (hereinafter also simply referred to as an “initiator” and an “initiation system”, respectively) in comparison with other conventional photosensitive layers.
However, when an image is drawn on such a photosensitive lithographic printing plate precursor with a laser beam, a region of insufficient polymerization is formed in the edge of image due to an inadequate exposure amount depending on an energy distribution profile of the laser beam. Thus, sharpness of the edge of image is impaired, resulting in decrease in resolution. Also, in the region of insufficient polymerization formed in the edge of image, defect of removal occurs depending on alkali concentration of a developer or condition of a developing brush in a development processing step to cause a large variation of halftone dot area in a printing plate formed.
Moreover, since such a lithographic printing plate precursor comprises a support having a roughened surface according to electrolytic treatment or brush treatment in order to ensure hydrophilicity, image quality and sharpness are additionally injured and reproducibility in the shadow area is severely degraded due to scattering of reflection light at the laser exposure.
On the other hand, requirements for the high definition AM screen printing or FM screen printing have recently increased in the field of CTP technique. Therefore, the resolution of lithographic printing plate precursor becomes an important performance.
The FM (Frequency Modulation) screen comprises fine halftone dots of approximately 20 microns irregularly arranged irrespective of screen angle and line number and expresses density gradation by halftone dot density per unit area. The features of FM screen print are that interference moire and rosette pattern do not occur, that tone jump in a halftone area having a halftone dot area ratio of approximately 50% is avoided, and that due to the small size of halftone dot, overlap of halftone dots decreases so that the color reproduced can be brilliantly seen.
In contrast to the FM screen, the AM (Amplitude Modulation) screen comprises halftone dots regularly arranged at a certain angle and expresses density gradation by halftone dot size per unit area. In Japan, a line number of the ordinary AM screen is 175 lines per inch. On the other hand, printing using a screen line number of 200 or more is defined as the high definition AM screen printing.
The characteristics of high definition print include decreases in the moire and rosette pattern, improvement in texture of image and improvements in feeling of reality and reproducibility of detail.
However, lithographic printing plate precursors accompanying the degradation of reproducibility in the shadow area, for example, the lithographic printing plate precursors having the polymerizable photosensitive layer described above are difficult to use for the purpose of providing printing plates suitable for the FM screen printing or high definition AM screen printing, because they cannot reproduce extremely fine halftone dots.
In JP-A-2003-43703, a photosensitive lithographic printing plate comprising a support having provided thereon an intermediate layer containing a polymer compound including a constituent unit having a sulfonic acid group in its side chain and a polymerizable photosensitive layer in order is described. However, the photosensitive lithographic printing plate is still insufficient for a printing plate precursor suitable for the high definition AM screen printing or FM screen printing. Particularly, since unevenness of halftone dot with the FM screen is severe, it is difficult to use the FM screen.
Also, it is commonly known that the polymerizable photosensitive layer is apt to decrease its sensitivity on polymerization inhibition due to oxygen present in the air. In order to prevent the decrease in the sensitivity, a protective layer is ordinarily provided on the polymerizable photosensitive layer. However, when oxygen permeability of the protective layer is decreased in order to prevent the polymerization inhibition due to oxygen, deterioration of developing property due to the polymerization in the dark is accompanied and it is difficult to consist with the preservation stability. In JP-A-2003-43703, although the attempt for the sensitivity to consist with the preservation stability was made by adding a polymerization inhibitor, the preservation stability was not sufficiently achieved.
In JP-A-2005-43658, a technique of using polyvinyl alcohol together with a vinyl caprolactam copolymer in a protective layer is described for the purpose of improving humidity dependence of a lithographic printing plate precursor having a photopolymerizable photosensitive layer. However, the lithographic printing plate precursor in JP-A-2005-43658 has a problem in that reproducibility in a shadow area is insufficient.
In JP-A-2006-113343, a lithographic printing plate material having a protective layer containing polyvinyl alcohol having a saponification degree of 85% to 100% and a water-insoluble cationic resin particle having Tg of less than 35° C. is described for the purpose of providing a lithographic printing plate material excellent, for example, in sensitivity, adhesion property of a protective layer or removability of the protective layer. However, the lithographic printing plate material in JP-A-2006-113343 also has a problem in that the reproducibility in a shadow area is insufficient.
In JP-T-2001-512251 (corresponding to WO 99/06890), a lithographic printing plate having a protective layer containing poly(1-vinylimidazole) or a copolymer of 1-vinylimidazole and at least one other monomer provided on a photosensitive layer is described for the purpose of improving oxygen impermeable property and adhesion property to an organic substrate. However, the lithographic printing plate in JP-T-2001-512251 (corresponding to WO 99/06890) has a problem of low sensitivity.
In JP-A-2002-221789, a technique of adding a nitrogen atom-containing heterocyclic photopolymerization intiator and a nitrogen atom-containing polymerizable monomer to a photosensitive layer is described for the purpose of providing a photopolymerizable lithographic printing plate improved, for example, in handling properties in a bright room and preservation stability. However, the lithographic printing plate in JP-A-2002-221789 has a problem in that the reproducibility in a shadow area and the sensitivity are insufficient.